Pulse oximetry is used to continuously monitor the arterial blood oxygen saturation of adults, pediatrics and neonates in the operating room, recovery room, intensive care units, and increasingly on the general floor. A need exists for pulse oximetry in the delivery room for monitoring the oxygen status of a fetus during labor and delivery, and for monitoring the oxygen status of cardiac patients.
Pulse oximetry has traditionally been used on patient populations where arterial blood oxygen saturation is typically greater than 90%, i.e., more than 90% of the functional hemoglobin in the arterial blood is oxyhemoglobin and less than 10% is reduced hemoglobin. Oxygen saturation in this patient population rarely drops below 70%. When it does drop to such a low value, an unhealthy clinical condition is indicated, and intervention is generally called for. In this situation, a high degree of accuracy in the estimate of saturation is not clinically relevant, as much as is the trend over time.
Conventional two wavelength pulse oximeters emit light from two Light Emitting Diodes (LEDs) into a pulsatile tissue bed and collect the transmitted light with a photodiode positioned on an opposite surface (transmission pulse oximetry), or an adjacent surface (reflectance pulse oximetry). The LEDs and photodetector are housed in a reusable or disposable sensor which connects to the pulse oximeter electronics and display unit. The "pulse" in pulse oximetry comes from the time varying amount of arterial blood in the tissue during the cardiac cycle, and the processed signals from the photodetector create the familiar plethysmographic waveform due to the cycling light attenuation. For estimating oxygen saturation, at least one of the two LEDs' primary wavelength must be chosen at some point in the electromagnetic spectrum where the absorption of oxyhemoglobin (HbO.sub.2) differs from the absorption of reduced hemoglobin (Hb). The second of the two LEDs' wavelength must be at a different point in the spectrum where, additionally, the absorption differences between Hb and HbO.sub.2 are different from those at the first wavelength. Commercial pulse oximeters utilize one wavelength in the red part of the visible spectrum near 660 nanometers (nm), and one in the near infrared part of the spectrum in the range of 880 nm-940 nm (See FIG. 1). Photocurrents generated within the photodetector are detected and processed for measuring the modulation ratio of the red to infrared signals. This modulation ratio has been observed to correlate well to arterial oxygen saturation as shown in FIG. 2. Pulse oximeters and pulse oximetry sensors are empirically calibrated by measuring the modulation ratio over a range of in vivo measured arterial oxygen saturations (SaO.sub.2) on a set of patients, healthy volunteers, or animals. The observed correlation is used in an inverse manner to estimate saturation (SpO.sub.2) based on the real-time measured value of modulation ratios. (As used herein, SaO.sub.2 refers to the in vivo measured functional saturation, while SpO.sub.2 is the estimated functional saturation using pulse oximetry.)
The choice of emitter wavelengths used in conventional pulse oximeters is based on several factors including, but not limited to, optimum signal transmission through blood perfused tissues, sensitivity to changes in arterial blood oxygen saturation, and the intensity and availability of commercial LEDs at the desired wavelengths. Traditionally, one of the two wavelengths is chosen from a region of the absorption spectra (FIG. 1) where the extinction coefficient of HbO.sub.2 is markedly different from Hb. The region near 660 nm is where the ratio of light absorption due to reduced hemoglobin to that of oxygenated hemoglobin is greatest. High intensity LEDs in the 660 nm region are also readily available. The IR wavelength is typically chosen near 805 nm (the isosbestic point) for numerical convenience, or in the 880-940 nm band where additional sensitivity can be obtained because of the inverse absorption relationship of Hb and HbO.sub.2. Unfortunately, pulse oximeters which use LED wavelengths paired from the 660 nm band and 900 nm bands all show diminished accuracy at low oxygen saturations.